Monday
نَوَيْتُ صَوْمَ يَوْمِ الْاِثْنَيْنِ سُنَّةً لِلّٰهِ تَعَالَى
Nawaitu shauma yaumil-itsnaini sunnatan lillaahi ta'aalaa
I intend to keep the recommended Monday fast, for Allah the Exalted.
Intention for Monday & Thursday Fasting
A recommended fast — deeds are raised on those days
Thursday
نَوَيْتُ صَوْمَ يَوْمِ الْخَمِيْسِ سُنَّةً لِلّٰهِ تَعَالَى
Nawaitu shauma yaumil-khamiisi sunnatan lillaahi ta'aalaa
I intend to keep the recommended Thursday fast, for Allah the Exalted.
When & how
Made at night, or even in the morning (before noon) as long as one hasn't eaten. The intention for a recommended fast is lighter than for an obligatory one.
For your little one
A lovely way to introduce voluntary fasting to an older child — start with half a day, then grow gradually.
Text follows the commonly taught wording (Shafi'i). Under teacher review.